Impact of dam-induced hydrological fluctuations on the behavior of the endangered scaly-sided Merganser (Mergus squamatus): implications for river regulation strategies and conservation.

Since the mid-20th century, the global expansion of dam and reservoir construction has profoundly altered river ecosystems, with cascading effects on channel morphology, flow regimes, and freshwater biodiversity. Among these impacts, the disruption of natural flow variability remains a critical but under-analyzed factor, particularly regarding its influence on riparian waterbirds. To address this gap, we investigated the behavioral responses of the endangered Scaly-sided Merganser (Mergus squamatus), a wintering diving duck, to dam-induced hydrological fluctuations downstream of the Taoyuan Dam in Hunan Province, China. Using scan sampling and Multivariate Bayesian Generalized Additive Models (GAMs), we assessed the relationships between bird activity patterns and water level dynamics. Our results reveal that increased water level and rate of change are associated with heightened foraging activity and a concurrent reduction in resting and preening. These shifts likely reflect changes in prey accessibility driven by fluctuating flow conditions. Based on these findings, we recommend modulating dam outflows to 3000 m³/s during peak foraging hours and reducing discharge to 2000 m³/s during midday and night. Our study underscores the potential of incorporating waterbird behavior into dam operation strategies for balancing ecological integrity with energy production.